Guide,
How to Clean Longboard Bearings the Right Way
Pull the wheels, lift the bearings out, clean them in a solvent that is not water, dry them, add a drop of oil and refit.
Taking the bearings out
Remove the axle nuts, slide each wheel off, and press the wheel down onto the end of the axle or a skate tool so the bearing pops out of its seat. Every wheel holds two bearings with a small spacer between them. Keep the spacers and washers beside the wheel they came from, because putting them back in the wrong order is the most common reason a freshly serviced board feels wrong.
If a bearing has a removable rubber shield, lift it with the tip of a pin. Sealed bearings that will not open can still be cleaned from the outside, only less thoroughly.
Cleaning and drying
Wipe off the loose grit, then let the bearings soak in a dedicated bearing cleaner or high-purity isopropyl alcohol and spin them in the liquid until the sound turns from gritty to smooth. Water is the one thing to avoid: it starts rust inside a steel race within days.
Lay the bearings out and let them dry fully. A hairdryer on low speeds this up, but a bearing that still smells of solvent is not dry yet.
Oil, refit and a quick check
Put in a small amount of skate bearing lubricant, just a few drops per bearing, then spin each one by hand. Too much oil collects dust and turns back into the paste you just removed.
Press the bearings back into the wheels, rebuild the axle with spacer and washers, and tighten the nut only until the wheel spins freely with no side wobble. Overtightening squeezes the bearing and slows the board more than dirt did.